The present invention relates to an image forming device including: toner image forming means for forming a toner image dealing with both colors and a black color to form color or monochromatic (black and white) images, respectively; an intermediate transfer belt onto which the toner image of the respective colors or the black color formed by the toner image forming means is transferred; and transfer means for transferring the toner image on the intermediate transfer belt onto a recording medium, and in particular to a color image forming device which is suitably miniaturized and can realize high image quality, further including a plurality of the toner image forming means per color, and arranging respective toner image forming means by a tandem method, while having the intermediate transfer belt for transferring the toner image.
Conventionally, major color image forming devices such as a color photocopying machine and a color printer have an arrangement in which a plurality of developing devices are provided, each of which forms a visible image (toner image) of a different color, and these toner images are eventually transferred one over another onto the same recording paper (transfer paper).
Generally, the color image forming device having the plurality of the developing devices is roughly classified into two types: a single photoreceptor type (or a single drum type); and a tandem type (multiple-step drum type).
First of all, in the single photoreceptor type color image forming device, a color image is formed by switching and successively operating the plurality of the developing devices capable of developing respective colors with respect to one photoreceptor drum. As a plurality of colors usually used therefor are, for example, the four colors of yellow (hereinafter referred to as xe2x80x9cYxe2x80x9d), magenta (xe2x80x9cMxe2x80x9d, hereinafter), cyan (xe2x80x9cCxe2x80x9d, hereinafter) and black (xe2x80x9cBkxe2x80x9d, hereinafter).
However, in the single photoreceptor type color image forming device, an image forming process (exposing, developing and transferring) is repeated for every color of the plurality of colors. For example, in the case of forming the color image with the four colors above, four developing devices are switched to be successively operated, so as to repeatedly perform image forming process for four times with respect to the photoreceptor drum.
Accordingly, in order to form one color image per one recording paper, four times a duration of time for image formation is necessary, thus requiring a considerable length of time for a whole image forming process. Therefore, in the single photoreceptor type color image forming device does arise a problem of the less counts of print per unit time, i.e., a slow image formation rate.
On the other hand, a tandem type color image forming device is made up of a plurality of toner image forming means having the photoreceptor drum and the developing device, each of which deals with each of the plurality of colors (four colors, for example), while juxtaposing these plurality of the toner image forming means per each color.
The following will describe one example of the tandem type color image forming device. For example, sequentially from an upper-stream side of a transport direction of a recording paper transport path for transporting recording paper which is a recording medium, respective image forming stations (toner image forming means) per respective colors of Y, M, C and Bk (black) are juxtaposed in a row (tandem), and in addition, a fixing device is provided on a downstream side of the recording paper transport path for these four image forming stations.
In the foregoing arrangement, first, recording paper which is static-absorbed with respect to the recording paper transport path is carried to the image forming station. Next, the toner images of the respective colors formed on respective photoreceptor drums are successively transferred one over another onto the recording paper. Thereafter, the recording paper after the transfer is carried to the fixing device in which fusion-pressurization fixation (fixing by applying heat and pressure) is performed on the toner image yet to be fixed on the recording paper, thus forming a color image on the recording paper.
Thus, in the tandem type color image forming device, the toner images of respective colors are formed by the image forming stations (toner image forming means) which are provided per color, and therefore, virtually, only a single step of the image forming process is required. Therefore, on a simple calculation, about four times a processing rate can be realized compared with the single photoreceptor type, and the problem of the slow image formation rate in the single photoreceptor type is cleared, thereby making it possible to realize a faster color image forming process.
Further, since, in the tandem type, as many photoreceptors and developing devices as types of color to be formed are needed, problems arise in terms of (i) an increase in the number of components, (ii) complication of manufacturing steps, and (iii) a rise in manufacturing costs. However, in recent years, an internal device has been miniaturized and composed in a unit (become a set device). This prevented the manufacturing steps from becoming more complicated and requiring higher costs, and the internal device has become relatively inexpensive. Accordingly, as far as the color image forming devices are concerned, the tandem type has been becoming the mainstream thereof, taking the place of the single photoreceptor type.
In the tandem type, however, the plurality of the image forming stations each of which is provided for each color are all set to be sequentially juxtaposed from the upper-stream side of one phase of the recording paper transport path. Accordingly, this causes the recording paper to be carried through as many transport paths as the plurality of the image forming stations. As a result, the transport path becomes long, which raises such a problem that the size of the tandem type color image forming device increases more than the single photoreceptor type.
Recently, therefore, there have been proposed various color image forming devices having the purpose of settling problems arisen in the single photoreceptor type and the tandem type, respectively. Typical techniques include color image forming devices disclosed, for example, in {circumflex over (1)} Japanese Unexamined Patent Publication No. 169175/1987 (Tokukaisho 62-169175 published on Jul. 25, 1987), {circumflex over (2)} Japanese Unexamined Utility Model Publication No. 5157/1991 (Jitsukaihei 3-5157 published on Jan. 18, 1991), {circumflex over (3)} Japanese Patent No. 2907944 (Publication No. Tokukaihei 4-13161 published on Jan. 17, 1992), {circumflex over (4)} Japanese Unexamined Patent Publication No. 341617/1993 (Tokukaihei 5-341617 published on Dec. 24, 1993) and {circumflex over (5)} Japanese Unexamined Patent Publication No. 271107/1995 (Tokukaihei 7-271107 published on Oct. 20, 1995).
First, the technique {circumflex over (1)} relates to a tandem type color image forming device, and as shown in FIG. 7, in an image forming section are juxtaposed four image forming stations 102a, 102b, 102c and 102d each of which deals with each color of Y, M, C or Bk, and besides, an intermediate transfer medium 103 having the shape of a belt is provided so that it commonly opposes to photoreceptor drums (not shown) in these image forming stations 102a through 102d. In other words, the four image forming stations 102a through 102d are disposed side by side on an upper side of the color image forming device, while on a lower side is disposed the intermediate transfer medium 103. The intermediate transfer medium 103 is rotatable in a direction of an arrow in the drawing.
In the foregoing arrangement, toner images having respective colors of Y, M, C and Bk are superimposed on a surface of the intermediate transfer medium 103. Thereafter, the toner images thus superimposed is transferred by a transfer device 104 onto a sheet of recording paper which was carried through a recording paper transport path, then, fixed on the sheet of recording paper by a fixing device 105.
In the technique {circle around (1)}, by suspending the intermediate transfer medium 103 having the shape of the belt in substantially a triangular shape, the transfer device 104 and fixing device 105 can appropriately be disposed in a spacing on the lower side of the device, without juxtaposing them as in the case of the image forming stations 102a through 102d. Consequently, the spacing within the device can be utilized efficiently, thus miniaturizing the color image forming device.
Next, the technique {circle around (2)} relates to a color image forming device taking a middle position between the single photoreceptor type and the tandem type, and as shown in FIG. 8, first, in the image forming section is used a photoreceptor having the shape of a belt (photoreceptor belt 113) to be suspended in a vertical direction with respect to a formation direction of a recording paper transport path 106, by suspension rollers 107. This photoreceptor belt 113 is rotatable in a direction of an arrow in the drawing.
Then, on a side of a direction which is an upper-stream side of a rotation direction of the photoreceptor belt 113 is disposed exposing means 108 (which exposes the photoreceptor belt 113 to form an electrostatic latent image thereon), and a Bk developing device 112d is disposed on the same phase and a downstream side of the exposing means 108, and further on a side of the other direction of the photoreceptor belt 113 are juxtaposed developing devices 112a, 112b and 112c per each color of Y, M and C. In addition, on a downstream side of the developing device 112c of C is disposed one suspension roller 107 to which the recording paper transport path 106 and transfer device 104 are connected.
In the foregoing arrangement, on a surface of the photoreceptor belt 113, from the upper-stream side thereof, the toner images of Bk, Y, M and C are successively formed by superimposing one over another. Then, the toner images which were superimposed one over another are transferred onto a sheet of the recording paper which was transported by the recording paper transport path 106. Thereafter, by the fixing device 105 is fixed the toner image which was transferred onto the sheet of the recording paper, thus forming a color image.
In the technique {circle around (2)}, since the photoreceptor belt 113 is suspended in a vertical direction with respect to the formation direction of the recording paper transport path 106, there is no need to juxtapose the respective developing devices 112a through 112d of Y, M, C and Bk, while, as in the case of the simple single photoreceptor type, it is not necessary to repeat a process for forming the toner image of each color for four times, thereby realizing both miniaturized and high-speed color image forming device.
Next, the technique {circle around (3)} also relates to a color image forming device taking a middle position between the single photoreceptor type and the tandem type. With this technique, as shown in FIG. 9, it is the same as the technique {circle around (2)} (or {circle around (1)}) that the photoreceptor belt 113 is used (or the intermediate transfer medium may be used instead) in the image forming section; however, other than the respective developing devices 112a through 112d of Y, M, C and Bk (or they may be the image forming stations instead), exclusive to a black and white image (monochromatic image), a fifth developing device 112e (or it can be an image forming station instead) for forming a toner image of Bk is included.
In the foregoing arrangement, when forming a color image, an image is formed in the first through fourth developing devices 112a through 112d corresponding to the four colors of Y, M, C and Bk, which is transferred onto a sheet of the recording paper by the transfer device 104 and fixed by the fixing device 105, while in the case of forming black and white image, a toner image of Bk is formed by the fifth developing device 112e alone, which is transferred onto a sheet of the recording paper by the transfer device 104 and fixed by the fixing device 105, thus miniaturizing the device while increasing a first copying speed of black and white image formation which is most frequently used in image formation.
Next, the technique {circle around (4)} also relates to a color image forming device taking a middle position between the single photoreceptor type and the tandem type. With this technique, as shown in FIG. 10, by using the intermediate transfer medium 103 which is used in technique {circle around (1)}, the image forming stations 102a through 102c of the respective colors of Y, M and C are disposed opposing to the intermediate transfer medium 103, while an image forming station 102d of Bk which is frequently used in image formation is disposed independently, apart from the image forming stations 102a through 102c of the other three colors.
In the foregoing arrangement, when forming a color image, the toner images of the respective colors of Y, M and C are tentatively transferred onto the intermediate transfer medium 103 before being transferred onto a sheet of the recording paper by the transfer device 104, and thereafter, in a Bk image forming station 102, a toner image of Bk is transferred onto a sheet of the recording paper and fixed by the fixing device 105. On the other hand, when forming a black and white image, an image is formed in the Bk image forming station 102d alone, skipping the intermediate transfer medium 103, and only the toner image of Bk is transferred onto a sheet of the recording paper and fixed by the fixing device 105, thereby miniaturizing the device on account of the intermediate transfer medium 103 while realizing faster image formation of a black and white image which is frequently used.
Further, the technique {circle around (5)} relates to a tandem type color image forming device, and moreover, it employs a high viscosity liquid developer for a color image, in which charged toner is dispersed within insulating liquid with high density.
In this technique, in addition to the arrangement in which the image forming stations of the four colors of Y, M, C and Bk are simply juxtaposed, as shown in FIG. 11, an arrangement is disclosed such that, in the image forming section, for example, an intermediate transfer medium having the shape of a belt is suspended in a horizontal direction, and the recording paper transport path 106 is formed in the vertical direction so as to oppose to the transfer device 104, then, the image forming stations of every two colors are juxtaposed in positions which are over and under the intermediate transfer medium 103. For example, in FIG. 11, the image forming stations 102a and 102b, and the image forming stations 102c and 102d are juxtaposed, respectively. Note that, in the example of FIG. 11, the recording paper is transported from down to up in the vertical direction, and thereby the fixing device 105 is disposed in an upper direction of the device.
With the foregoing arrangement, an image forming device can be miniaturized because a device width of a whole color image forming device is not more than a total width of the image forming stations 102a and 102b (or 102c and 102d), while a color image forming device capable of faster image formation, high resolution and less pollution can be provided because of the use of the liquid developer.
However, any one of the color image forming devices of the respective arrangements above raises a problem of not being capable of fully realizing miniaturization of the device and the faster image formation.
First, in the technique {circle around (1)}, the Bk image forming station 102d is juxtaposed with the image forming stations 102a through 102c of the other colors. Therefore, as seen in problems to be solved in the techniques {circle around (3)} and {circle around (4)}, there arises a problem of difficulty in realizing faster black and white image formation which is frequently used.
Further, in the technique {circle around (2)}, as in the technique {circle around (1)}, it is difficult to realize faster black and white image formation. More specifically, in an electrophotographic color image forming device, in order to adjust itself to highly frequent black and white image formation, not only simply faster image formation speed but also a larger toner storing capacity of a Bk developing device (or image forming station), i.e., the larger size of the Bk developing device are necessary.
However, in the technique {circle around (2)}, due to the arrangement, the exposing means 108 requires to be juxtaposed with either one of the developing devices 112a through 112d, and moreover, in order to increase the size of the Bk developing device 112d while avoiding increasing the size of a whole device, it is necessary to dispose the exposing means 108 and Bk developing device 112d on a side of the same plane of the photoreceptor belt 113. This, therefore, results in disposing the Bk developing device 112d of all the developing devices 112a through 112d on the most upper-stream side, which causes difficulty in realizing faster black and white image formation.
Further, in the technique {circle around (3)}, while realizing faster black and white image formation, the fifth developing device 112e exclusive to a black and white image is provided, other than the Bk developing device 112d for a full color. As a result, twice as many the Bk developing device or the image forming station should be provided, which results in increasing the number of components and costs accordingly, complicating manufacturing steps, and increasing capacity of a color image forming device, and thereby preventing a whole device from being sufficiently miniaturized.
On the other hand, in the technique {circle around (4)}, in order to realize faster black and white image formation, the Bk image forming station 102d is separately provided, apart from the image forming stations 102a through 102c of the other colors formed by integrating them with the intermediate transfer medium 103. Accordingly, it is not necessary to provide twice the number of the Bk image forming stations, and it is possible to increase the size of the Bk image forming station 102d, and further, unlike the technique {circle around (3)}, the problems such as the largely increased number of components and increased capacity can be prevented. However, the suspension of the intermediate transfer medium in the vertical direction causes a problem of increasing the vertical direction (height) of the device.
Further, in the technique {circle around (5)}, since every two of the image forming stations 102a and 102b or 102c and 102d are disposed over and under the horizontally suspended intermediate transfer medium 103, as in the case of the technique {circle around (1)}, in effect, it becomes similar to a state in which the Bk image forming station 102d is juxtaposed with the image forming stations 102a through 102c of the other colors. As a result, there arises such a problem that realizing faster black and white image formation is difficult.
Further, the respective color image forming devices above have an arrangement including a plurality of image forming stations disposed side by side, or an arrangement including a plurality of developing devices which are juxtaposed as in the manner of an intermediate type between the single photoreceptor type and the tandem type; however, such arrangements may result in deterioration of image quality, which is called a back-transfer phenomenon. This back-transfer phenomenon is such that unfixed toner which was tentatively transferred onto a sheet of the recording paper (or the intermediate transfer medium) in the plurality of the image forming stations (or developing devices) is caught by an image forming station disposed on a closest downstream side, and therefore, a desirable color image is not formed.
The foregoing back-transfer phenomenon will be explained more specifically with reference to, for example, the technique {circle around (1)} shown in FIG. 7. In the case of forming a color image of the four colors of Y, M, C and Bk, first, a toner image of a first color Y, which was formed in a developing process of the Y image forming station 102a of the most upper-stream, is transferred onto a sheet of the recording paper. The recording paper having the toner image of Y transferred thereon is transported to a position of the next image forming station 102b of M, and a toner image of a second color M which was formed in a developing process of the M image forming station 102b is transferred onto a sheet of the recording paper by superimposing one over another.
However, when transferring the M toner image, a portion of the unfixed Y toner image previously transferred is reverse-transferred with respect to a photoreceptor (not shown) of the M image forming station 102b. The same phenomenon can be seen in the cases of transferring a toner image of a third color C and of transferring a toner image of a fourth color Bk.
To this end, on the recording paper after the toner image of the fourth color Bk was transferred in the Bk image forming station 102d of the most downstream, the amount of the adhering toner of the toner image of the first color Y shows a decrease of tens of percent against an original adhesion amount, due to an occurrence of the back-transfer phenomenon. Such decrease in the adhesion amount is also shown in the cases of the toner images of the second color M and the third color C.
Namely, the earlier a toner image is transferred, the less the amount of toner to adhere becomes, compared with a desired amount of toner to adhere. Consequently, when, for example, forming the respective toner images of four colors so as to transfer them onto the recording paper in one image forming process, assuming that toner of the respective colors each having the same amount as the others is transferred and adheres with respect to the recording paper, it would essentially satisfy xe2x80x98Y (a first color) adhering amountxe2x80x99 xe2x80x98M (a first color) adhering amountxe2x80x99=xe2x80x98C (a first color) adhering amountxe2x80x99=xe2x80x98Bk (a first color) adhering amountxe2x80x99. However, when the back-transfer phenomenon occurs, a relation among them becomes xe2x80x98Y (the first color) adhering amountxe2x80x99 less than xe2x80x98M (the first color) adhering amountxe2x80x99 less than xe2x80x98C (the first color) adhering amountxe2x80x99 less than xe2x80x98Bk (the first color) adhering amountxe2x80x99, which results in an unbalanced color scheme.
In the foregoing case, for example, since the toner amount of Y as the first color becomes the least of all, one feels that a Y component in a whole image is thinner than the other colors. This causes reduction in chroma in the case of using a single Y-component, and moreover, it even causes change in a hue in the case of using a color formed by a mixture of Y. The same phenomenon is observed in the cases of M as a second color and C as a third color, i.e. the earlier a color is transferred, the thinner the density becomes, and relatively, the later a color is transferred, the thicker the density becomes. Thus, the balance of a color scheme of a color image as a whole largely deteriorates in comparison with that of an original color image.
Causes of the foregoing back-transfer phenomenon include the following. Namely, when toner from a developing roller adheres to the photoreceptor (development is performed) in accordance with an image voltage of the photoreceptor (drum or belt) in the image forming process, not all the toner is always charged with a uniform voltage. Therefore, toner adhering to the photoreceptor includes toner having a weak polarity and toner having a charge of a reverse polarity mixed therein. Furthermore, there may be a case where an electrical discharge occurs in a process of transferring the toner image onto the recording paper so as to remove the recording paper from the photoreceptor. This electrical discharge causes to generate toner which is to have the reverse polarity later.
In case where a toner image including these toner having the weak polarity and toner having the reverse polarity receives a transfer charge in a transfer process of the following stage, when the toner has a normal charge, the transfer charge does not change the polarity of the toner, and repulsive force acts between the toner on the recording paper and photoreceptor so as to maintain the toner to adhere to the recording paper, while, in the case of the toner having the reverse polarity, the transfer charge also has the reverse polarity, and therefore, electrostatic attraction force acts from the recording paper to a side of the photoreceptor, thus the toner having the reverse polarity returns from the recording paper to the side of the photoreceptor.
In view of the foregoing problems, it is an object of the present invention to provide a color image forming device capable of (1) reducing the size of an entire device, (2) increasing the size of an image forming station of a black color which is frequently used in image formation, or a developing device, (3) increasing the speed of an image forming process, particularly the speed of a black and white (monochromatic) image forming process which is frequently used in image formation, and (4) suppressing occurrence of a back-transfer phenomenon so as to effectively form a color image with superior color reproducibility.
Namely, in order to attain the foregoing object, the color image forming device according to the present invention includes a plurality of image forming stations for respectively forming toner images of different colors, and an intermediate transfer belt on which the toner images which are respectively formed by the plurality of image forming stations are successively transferred one over another, the image forming device further including, as the plurality of image forming stations, one or more chromatic color image forming stations for forming a toner image having a chromatic color, and only one black image forming station for forming a toner image having a black color, and the intermediate transfer belt being suspended in a shape of a substantially flat plate by at least two suspension rollers inside a main body, and the chromatic color image forming stations being disposed on one side of the suspended intermediate transfer belt, and the black image forming station being disposed on the other side of the intermediate transfer belt, alone and on a most downstream side of all the image forming stations.
With the foregoing arrangement, with respect to the chromatic color image forming stations, the black image forming station is virtually provided independently. Consequently, not all the plurality of image forming stations are to have a tandem disposition, thereby suppressing increase in size of the entire device and increasing the size of the image forming station of the black color which is frequently used in image formation.
Moreover, when forming a monochromatic image, processing speed of the black image forming station can be increased, thereby further increasing first copying speed of monochromatic image formation and increasing speed of monochromatic image formation.
In the color image forming device, the black image forming station has a shape of a substantially flat plate which extends two-dimensionally, and is disposed so that a direction of the two-dimensional extension coincides with a suspending direction of the intermediate transfer belt.
With the foregoing arrangement, since the black image forming station itself is formed in a shape of the substantially flat plate, the shape coincides with a shape of the intermediate transfer belt in the suspending direction. Therefore, the black image forming station can be disposed along the suspending direction of the intermediate transfer belt, thereby making effective use of space inside a main body.
In the color image forming device, it is preferable that the black image forming station includes a substantially cylindrical photoreceptor drum for carrying an electrostatic latent image, a developing device for developing the electrostatic latent image into a toner image with toner, and a developer tank for storing toner, wherein the photoreceptor drum, the developing device and the developer tank are disposed side by side in one direction in this order, which takes a form of the substantially flat plate.
With the foregoing arrangement, since it is possible to form the developer tank and the developing device to extend in a form of a flat plate, a storing capacity of toner can be increased in size so as to store the amount of toner corresponding to monochromatic image formation which is frequently used in image formation.
In the color image forming device, inside the main body of the device, the intermediate transfer belt is suspended in a substantially horizontal direction, and a sheet feeder cassette for storing a recording medium on which a toner image is finally transferred is disposed along the suspending direction of the intermediate transfer belt, and the black image forming station is disposed in a spacing having a shape of a substantially flat plate between the intermediate transfer belt and the sheet feeder cassette.
With the foregoing arrangement, the black image forming station which is disposed alone is disposed in the spacing having the shape of the substantially flat plate between the intermediate transfer belt and the sheet feeder cassette. Therefore, the black image forming station can be stored with no space, thereby effectively using a spacing having the shape of the substantially flat plate inside the main body of the device.
Particularly, this spacing having the shape of the substantially flat plate, though lacking a three-dimensional extension (the size in a direction of height of a device main body), has a sufficient two-dimensional extension (the size in a horizontal direction). Therefore, by forming the developing device and the developer tank of the black image forming station in a shape of a flat plate, these can be increased in size, and the developing device can use, as developer of Bk, two-component developer including toner and carrier as main components.
The color image forming device further includes a transfer roller for finally transferring a toner image, which has been transferred onto the intermediate transfer belt, to a sheet of recording paper, a fixing device for fixing the toner image, which was finally transferred, on the sheet of recording paper, and a recording paper transport path for transporting the sheet of recording paper from a sheet feeder cassette to the fixing device via the transport roller, wherein: the intermediate transfer belt is suspended in a substantially horizontal direction inside the main body, and the recording paper transport path is disposed in a substantially vertical direction, and the sheet feeder cassette is disposed in a lower part of the main body.
With the foregoing arrangement, the fixing device which is to reach a high temperature is disposed in an upper position, thus making it possible to increase a spacing between the intermediate transfer belt and the fixing device, thereby preventing fusion of toner into the intermediate transfer belt due to an influence of the heat of the fixing device, while, in the case of having a cleaning device, preventing decay of a blade of the cleaning device.
Further, the color image forming device preferably has an arrangement in which the sheet feeder cassette includes a feeder roller for feeding the recording paper in the recording paper transport path, and the black image forming station is disposed in a spacing having a shape of a substantially flat plate between the intermediate transfer belt and the sheet feeder cassette so that a photoreceptor drum comes closer to the feeder roller.
With the foregoing arrangement, the photoreceptor drum which has a relatively small capacity is disposed on a side of the feeder roller which is a projecting member, while the developer tank which requires a larger capacity is disposed in a position which is opposite to the feeder roller which is the projecting member, thereby making effective use of the space having the form of the substantially flat plate, while increasing the capacity of the developer tank in the black image forming station which is more frequently used in image formation, without increasing the size of the entire device in the direction of height.
In the color image forming device, it is preferable that a diameter of the photoreceptor drum included in the black image forming station is formed to be larger than a diameter of a photoreceptor drum included in the chromatic color image forming station.
With the foregoing arrangement, since, in the black image forming station, the diameter of the photoreceptor drum is large, thereby suppressing reduction in a film of a photosensitive film due to variation with time, and realizing a longer life cycle. Consequently, when forming a toner image of a black color which is frequently used in image formation, a longer period of time for use of a photoreceptor drum before replacement (reducing the number of time for replacement), and increasing ease of use.
In the color image forming device preferably includes either one of a laser scan unit and a light-emitting diode array so as to allow the plurality of image forming stations to expose a surface of an image carrier for carrying an electrostatic latent image so as to form the electrostatic latent image.
With the foregoing arrangement, both laser scan unit (LSU) and light-emitting diode array (LED array) are to be suitably used in image formation based on digital information, in a particular case where the color image forming device of the present invention is a digital color image forming device, a high-quality color image can be formed.
In the color image forming device, it is preferable that the chromatic color image forming station includes the light-emitting diode array.
With the foregoing arrangement, since the chromatic color image forming stations has the tandem disposition, the size of the device main body in a vertical direction (direction of height) can be greatly reduced when using the light-emitting diode array (LED array) which becomes much smaller than the LSU and the like.
The color image forming device preferably includes a cleaning device for cleaning residual toner on the intermediate transfer belt, wherein: the intermediate transfer belt which is suspended in the shape of the substantially flat plate is suspended by two suspension rollers which are disposed adjacently in a substantially vertical direction, and by one suspension roller which is disposed in a position apart from the two suspension rollers, and the cleaning device is disposed so as to face the intermediate transfer belt which is suspended in a substantially vertical direction by the two suspension rollers which are disposed adjacently.
With the foregoing arrangement, since the feeder roller, the suspension roller and the transfer roller can be disposed adjacently, space can be utilized more efficiently. Moreover, since the cleaning device is disposed on a substantially vertical side between the adjacently disposed suspension rollers, it is unnecessary to dispose the cleaning device next to the image forming station side by side, thereby reducing space of the device main body in a direction of both sides.
Further, the cleaning device is disposed on a substantially vertical portion, and therefore, when cleaning residual toner on the intermediate transfer belt, the residual toner which is scraped with a blade, etc., can be received in a lower part, thus cleaning the residual toner by utilizing the own weight of toner.
In the color image forming device, it is preferable that one of at least two suspension rollers suspending the intermediate transfer belt is disposed upper than the other suspension roller with respect to a horizontal direction so as to suspend the intermediate transfer belt with a tilt.
With the foregoing arrangement, making it possible to reduce the size of a spacing between a plurality of exposing means and to increase a capacity of a developer tank of the black toner image forming means, the size of a spacing of installation can also be reduced, thereby reducing the size of the image forming device in a width direction, and reducing the size of the spacing of installation.
In the color image forming device, it is preferable that the black image forming station uses two-component developer including toner and carrier, and the chromatic color image forming station uses one-component developer including toner.
With the foregoing arrangement, in the chromatic color image forming station, one-component developer is sued, thereby reducing the size of the chromatic color image forming station, and increase in size of the device main body in a horizontal direction can be suppressed. On the other hand, the black image forming station uses two-component developer, thereby realizing a longer life cycle of the developing device and increasing the speed of forming a black toner image.
Further, in order to solve the foregoing problems, the image forming device according to the present invention includes a plurality of image forming stations for forming and transferring toner images onto an intermediate transfer belt are disposed side by side along a transport or moving direction of the intermediate transfer belt so as to allow the plurality of toner images to be successively transferred one over another with respect to the intermediate transfer belt, and the plurality of image forming stations have an intermediate transfer roller for transferring the formed toner images to the intermediate transfer belt, and a transfer voltage of the intermediate transfer roller in an image forming station on a downstream side of the transport or moving direction is set to be lower than a transfer voltage of the intermediate transfer roller in an image forming station on an upstream side.
With the foregoing arrangement, the transfer voltage of a toner image is decreased toward a downstream side. Therefore, even when a preceding toner image includes reversely charged toner or weakly charged toner, an electrostatic attraction force acting between the reversely charged toner or weakly charged toner and the photoreceptor drum also becomes smaller. As a result, back-transfer of toner from the intermediate transfer belt to a surface of the photoreceptor drum is suppressed, thus effectively preventing occurrence of a back-transfer phenomenon in the image forming stations of the downstream side, thereby forming a high-quality image.
Alternatively, the present invention may be adopted so that the charged quantity of toner used in the image forming station of the downstream side in the transport or moving direction is set to be lower than the charged quantity of toner used in the image forming station of the upstream side.
Here, the charged quantity of toner is preferably set by at least one of the following methods: a method of selecting the type of charge controlling agent, a method of selecting the type of external additives of toner, a method of changing the amount of charge controlling agent added, a method of changing the dispersed state of charge controlling agent, and a method of varying the pressure of contact of the doctor blade for charging toner.
With this arrangement, the charged quantity of developer, i.e., toner is decreased toward the downstream side. Thus, even when the preceding toner image include reversely charged toner or weakly charged toner, the transfer voltage applied to transfer the later toner image can be decreased, and the electrostatic attraction force acting between the reversely charged toner or weakly charged toner and the photoreceptor drum also becomes smaller. As a result, back-transfer of toner from the intermediate transfer belt to the surface of the photoreceptor is prevented, thus effectively preventing occurrence of the back-transfer phenomenon in the image forming stations of the downstream side, thereby forming a high quality image.
Alternatively, the present invention may be adopted so that the image forming station includes the photoreceptor drum for carrying an electrostatic latent image, and a developing device for developing the electrostatic latent image which is carried on the photoreceptor drum, and the developing potential of the developing device of the image forming station of the downstream side of the transport or moving direction is set to be lower than the developing potential of developing device of the image forming station of the upstream side.
Here, the developing potential is preferably set by the method of varying the applied voltage to the developing device while holding the electrostatic latent potential constant, or by the method of varying the electrostatic latent potential while holding the applied potential to the developing device constant.
With this arrangement, the developing potential |VLxe2x88x92Vbias| used to develop the electrostatic latent image on the photoreceptor drum with toner is made smaller toward the downstream side. Thus, the electrostatic attraction force acting between the reversely charged toner or weakly charged toner and the photoreceptor also becomes smaller. As a result, back-transfer of toner from the intermediate transfer belt to the surface of the photoreceptor drum is prevented, thus effectively preventing occurrence of the back-transfer phenomenon in the image forming stations of the downstream side, thereby forming a high quality image.
Alternatively, the present invention may be adopted so that the particle size of toner used in toner image forming means of the downstream side in the transport or moving direction is set to be larger than the particle size of toner used in image forming station of the upstream side.
With this arrangement, the particle size of developer, i.e., toner is made larger toward the downstream side. Thus, the electrostatic attraction force required for toner movement becomes relatively larger. Therefore, even when the preceding toner image includes reversely charged toner or weakly charged toner, toner movement is prevented even when there is an electrostatic attraction force which would move normal toner in transfer of the later toner image, thus reducing the electrostatic attraction force acting between the reversely charged toner or weakly charged toner and the surface of the photoreceptor drum. As a result, back-transfer of toner from the intermediate transfer belt to the surface of the photoreceptor drum is prevented, thus effectively preventing the back-transfer phenomenon in the image forming stations of the downstream side, thereby forming a high quality image.
Therefore, the image forming device in accordance with the present invention has at least any of the foregoing arrangements for preventing occurrence of the back-transfer phenomenon, and it very preferably has an arrangement including one or more chromatic color image forming stations for forming a chromatic toner image and a single black image forming station for forming a black toner image as the plurality of image forming stations, and in which an intermediate transfer belt is suspended in a horizontal direction in the main body of the device, wherein the chromatic color image forming stations are disposed side by side on one side of the intermediate transfer belt, and the black image forming station is independently disposed on the other side of the intermediate transfer belt and on the most downstream side of all the image forming stations.
With this arrangement, it is possible to prevent increase in size of the entire device, to increase the size of the image forming station of Bk which is frequently used in image formation to adapt to large counts of monochromatic (black and white) image formation and to independently dispose the single image forming station of Bk, and therefore to increase the first copying speed of monochromatic (black and white) image formation which is frequently used in image formation, and to prevent occurrence of the back-transfer phenomenon of toner to efficiently obtain a color image with superior color reproducibility, thereby obtaining a small color image forming device with superior image quality and fast image forming speed.
That is, the color image forming device according to the present invention is arranged to include a plurality of image forming stations for forming and transferring toner images onto the intermediate transfer belt, which are disposed side by side along the transport or moving direction of the intermediate transfer belt so as to allow the plurality of toner images to be successively transferred one over another with respect to the intermediate transfer belt, wherein assuming that a toner image to be first transferred onto the intermediate transfer belt is a preceding toner image, and a toner image to be transferred after the preceding toner image is a later toner image, when at least the preceding toner image includes improperly charged toner, a control is performed to reduce chargeability of the later toner image when transferring the later toner image.
Further, in the color image forming device according to the present invention, when the preceding toner image does not include improperly charged toner, a control may be performed to improve chargeability of the later toner image when transferring the later toner image.
In any of the foregoing arrangement, when the image forming stations are in the tandem disposition, the chargeability of the later toner image is changed when transferring the later toner image by the image forming station of the downstream side. Further, when there is no improperly charged toner, the chargeability of the later toner image at the time of transfer is improved to make the toner image to be attracted to the intermediate transfer belt more easily. As a result, it is possible to effectively prevent occurrence of the back-transfer or re-transfer phenomenon.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.